System to Improve Icing Problems in Blast Freezer Tunnels

ABSTRACT

A method and device for chilling food products by jets blasted into a tunnel, said tunnel comprising/a conveyor for conveying products into a tunnel, at least one plate, located opposite the conveyor, and provided with through holes, the plate or plates being located above and/or below the conveyor; and at least one means of blowing a cold fluid through the through holes and towards the products, characterized in that, during all or part of the periods of operation of the tunnel and/or during all or part of the periods of stoppage of the tunnel, there takes place an operation to clear all or part of said through holes, using a network of channels allowing a blow gas to be distributed opposite all of said through holes.

The present invention concerns the field of methods for chilling food products in apparatus of the tunnel type, using direct or indirect injections of a cryogenic fluid such as liquid nitrogen.

Traditionally, such tunnels comprise:

-   -   an insulated chamber provided with an entry and an exit;     -   means for conveying products between the entry and the exit;     -   means for bringing a cryogenic liquid into the chamber to enable         the products to be put in contact directly with this liquid         (usually, these feed means comprise manifolds for spraying         liquid onto the products) or indirectly through the fact that         the cryogen is injected into exchangers internal to the tunnels         (usually referred to in this art as “cold batteries”), the         transfer of the cold to the products being effected by an         exchange with the internal air of the tunnel through the action         of ventilation means associated with each battery;     -   and ventilation means, able to blow cold gas onto the passing         products.

The food industry is continuously seeking equipment that is more and more efficient economically and in particular more and more compact (using the least surface area on the ground). In many cases this makes it possible to increase the production capacity of a given site without investing in new buildings. Recent freezer tunnels thus offer more and more elaborate techniques for increasing the production capacities whilst reducing the footprint of the equipment.

To do this, recent high-performance equipment must increase the coefficient of heat transfer with the product to be frozen. In the case where cold-gas ventilation is used to transfer the cold from the cold source to the product, a common technique consists of increasing the velocity of this cold gas. The cold gas is then pressurised and injected in the form of jets impacting directly on the product (“impingement” is often spoken of this industry). The heat transfer coefficient is then very high and the refrigerating capacity of the machine per unit surface area is also very high. The document EP 1 449 443 A1 illustrates this impaction prior art.

This very interesting technique does however have drawbacks and technical difficulties. The ventilation power is often difficult to control especially after several hours of production, when the characteristics of the ventilation system have been modified by contamination thereof (frost or other deposit).

Moreover, the air flow and the ventilation velocities are very high, and asymmetry in the distribution of the cold gas is observed, which causes entries of air on one side of the equipment and exits of air on the other side.

It has been demonstrated that the frost results mainly from infiltrations of outside air. These infiltrations of air from the production room into the freezer tunnel are of course accompanied by an entry of water vapour (moisture from the air) that will be deposited in the form of frost inside the machine. Thus this poor control of the equilibrium of gases causes not only an overconsumption of refrigeration and an additional production cost but also accelerated contamination of the system for ventilating the plates producing the blast jets and possibly the exchangers installed, which causes self-amplification of the phenomenon.

Various solutions have been proposed in the literature for improving this excessive-frosting situation by limiting the air inlets and in particular:

-   -   the establishment of buffer zones at the tunnel entry/exit: this         solution sets out to combat one of the phenomena observed in         such tunnels where, when the jets impact at high speed on the         sole plate, some of the cold air is expelled out of the tunnel,         the quantity of air in the tunnel being constant, this         automatically creates infiltrations of warm air. The area where         the products are loaded into the tunnel then acts as a “tube”         that guides the air flow but does not significantly reduce         infiltrations of air.

Then one (or more) buffer areas are positioned, immediately downstream of the loading area in one case and immediately upstream of the tunnel exit in the second case, buffer zones that are not ventilated, through enlargements of the external shell of the tunnel (from 50 cm to 1.5 m for each zone at the entry and exit of the freezer), thus creating on each side a dead space that dampens the gas velocities, which naturally reduces the entries of air and exits of cold gases. However, this system has a major drawback, which is increasing the overall size of the machine, which runs counter to the effect sought initially by the adoption of the blast jets.

-   -   the use of physical curtains: this solution consists of placing         plastic or stainless steel curtains at the entry/exit of the         apparatus. These curtains in fact limit the entries of air and         the exits of cold gases. However, while they are more effective,         they consequently block the entries to the freezer. In practice,         they are often little effective since, being situated on the         product passage, they cannot really close off the entries/exits.         In addition, they sometimes pose hygiene problems when they         touch the passing products.     -   the use of gaseous curtains: this solution consists of         installing, at the tunnel entry and exit, a system for blowing         air intended to compensate for the imbalance in the tunnel. This         system may sometimes give satisfactory results when the         imbalance is very small but on the other hand, when it is a case         of compensating for average to high imbalance, this system has         not yet shown its ability to re-establish equilibrium of the         cold gases.

Nevertheless, in this food industry, it is necessary to bear in mind the fact that one of the origins of the entries of moisture is also related to the evaporation of some of the water contained in the incoming product itself, and particularly certain incoming products. Thus the frost appears more or less rapidly at the blast orifices but it always appears, and the flow rate of injected gas decreases over time just like the thermal efficiency of the machine.

It is then known that other types of solution have been proposed in the literature, and in particular:

-   -   adding a vibration source on the plates that support the         orifices. If the vibrations are sufficiently powerful to detach         the frost, this technique may prove to be advantageous.

However, these powerful vibrations do in themselves present several drawbacks: they weaken the machine (the welds, the chassis, but also the electrical components such as motors, temperature probe, sensors, etc.). It must also be mentioned that the system generating vibrations may have insufficient reliability when it operates in a very cold environment, and in addition the cost of the system is fairly high.

-   -   the solution that consists of creating overpressures from time         to time in the system supplying the orifices. This overpressure         is capable of removing from the orifices the frost that         encumbers them if the pressure is sufficiently high. The         drawback of this technique is the cost thereof: this is because         it requires the use of an oversized ventilation system capable         of temporarily delivering high power.     -   the use of an electrical system for locally heating the         orifices, but such use proves in practice to be very difficult.

It will therefore be understood that, in order to preserve a good level of heat exchange in this type of machine, it would be advantageous to be able to leave the blast orifices clear without needing to stop or heat the machine.

As will be seen in more detail hereinafter, the present invention sets out to propose a novel solution for preventing the clogging by formation of frost at the orifices forming the blast jets, without the use of a mechanical vibration or heating, through a system for distributing a blow gas (for example air) that is well designed and sized.

To this end, the invention proposes a device for cooling food products by blast jets comprising a tunnel that comprises:

-   -   a conveyor for the products in the tunnel     -   at least one plate, situated facing (above and/or below) the         conveyor, and provided with through orifices,     -   and at least one means for blowing a cold fluid by blasting         through traversing orifices and towards the products,

the device comprising a network of channels for distributing a blow gas above the top plate and/or below the bottom plate, provided with through orifices, all or some of the channels being provided with injection orifices or injection tubes, the positioning of which in the network directs blow gas towards the through orifices in the plate or plates, in order to simultaneously clear all or some of these through orifices.

As will have been understood from a reading of the above, the blowing is thus effected on the side where the frost accumulates: this is because in practice, in such a blast tunnel, the frost accumulates above the top plate and below the bottom plate (the one situated under the conveyor as projecting cold gas upwards in order to impact on the bottom face of the products), that is to say the blowing thus takes place on the side of the arrival of the cold gases in the system (rather than on the side where the blast jets emerge).

Though it can be envisaged according to the invention clearing some of the orifices (for example 80% to 90% of these orifices), it will be preferred to position a blowing network making it possible to clear all the through orifices.

In the device of the invention, the plate or plates provided with through orifices are advantageously made from food-quality stainless steel. These plates are removable so as to facilitate cleaning thereof after operation.

In this industry plates of very varied forms are encountered, and in particular these may be planar or in a V shape or corrugated.

For purely illustrative purposes, the through orifices in the plate may also be of varied forms and in particular in the form of cylinders, circles, slots, optionally beveled (individual ones distributed along the plates or several slots distributed over the length of the plate, each slot occupying almost all the width of the plate), or cones with beveled or rounded edges.

Usually but non-limitatively, the fluid-blowing means is a centrifugal fan driven by a motor.

Likewise, the tunnel may use pairs of plates, a top plate and a bottom plate, situated in parallel on either side of the conveyor, at least one, and preferably both, being provided with through orifices.

The invention also concerns a method for cooling food products by blast jets in a tunnel, a tunnel that comprises:

-   -   a conveyor for the products in the tunnel     -   at least one plate, situated opposite the conveyor, and provided         with through orifices, the plate or plates being situated above         and/or below the conveyor;     -   and at least one means for blowing a cold fluid through the         through orifices and towards the products,

the method being characterised in that, throughout all or some of the operating periods of the tunnel and/or during all or some of the stoppage periods of the tunnel, an operation is carried out of clearing all or some of said through orifices, in the following manner:

-   -   a network of channels is available for distributing a blow gas         above a top plate and/or below a bottom plate that is provided         with through orifices, all or some of the channels being         provided with injection orifices or injection tubes, the         position of which in the network makes it possible to direct         blow gas towards the through orifices in the plate or plates,     -   said network of channels is fed with blow gas in order to         simultaneously clear all or some but preferably all said through         orifices.

The accompanying FIG. 1, in the various views (a), b), c) etc.) thereof illustrates, by means of schematic partial representations, an embodiment of the invention where:

-   -   the impacting plate is in the form of a corrugated stainless         steel sheet in the form of successive Vs, each V being provided         with a row of slots/orifices, optionally beveled;     -   FIG. 1 a) displays the structure of the plate with 7 Vs and the         blowing device, in exploded mode for better reading (the         right-hand view in this FIG. 1 a) provides the detail of a V);     -   FIG. 1 b) displays the plate and the blowing device in assembled         mode;     -   FIG. 1 c) displays a situation where the through orifices are         obstructed by frost;     -   while FIG. 1 d) displays the effect of the arrival of a jet of         compressed air in each V making it possible to clear each         through orifice (here once again the right-hand view provides         the detail of a V);     -   as can be seen clearly in these figures, for this embodiment         there is a service tube, closed at one of the ends thereof, with         a diameter of around 40 mm, that runs along the end of the Vs on         one side of the plate, and facing each V there is a small         injector or tube connected to the service tube (here with an         inside diameter of around 2 to 5 mm) that injects compressed air         in the V. The injected air will therefore first of all touch the         first orifice and clear it, then the second, then the third,         etc. Thus an injector unblocks a line of several orifices over a         distance of approximately 200 to 600 mm depending on the         pressure and flow rate of air injected.     -   when injection is demanded (as has been seen, whether this be         during a production phase of the tunnel or during a period where         it is stopped, and whether this be on isolated command by an         operator or at pre-programmed regular intervals, or on the         instruction of an automatic controller following the reception         of information representing a blocked state of the orifices, for         example a pressure measurement in the ventilation) the         accumulation of frost situated facing each injection tube is         swept by the power of the air jet, the detached frost is carried         away by the air flow, and the tunnel can continue to operate         normally.     -   the injections of blow gas may be very short because of the         pressures envisaged (typically a few seconds, for example         between 0.2 and 2 seconds).

The consumption of compressed air may be optimised by spacing the injections apart as far as possible while preserving satisfactory thermal efficiency of the machine between the injections.

In order to limit the number of compressed air injectors, the orifices are aligned and the injection of compressed air is arranged so that it impacts a line of several orifices.

As seen above, for this embodiment, a single service tube is installed disposed on one side of the plate, but obviously it is clearly possible to envisage, depending in particular on the size of the impacting plate, to use several service tubes, one on each side of the plate, and blowing then takes place in the two opposite directions.

Even larger plates that would require more than two service tubes (one at each end and one for example at the middle of the plate) are sufficiently rare not to dwell here on this situation.

FIG. 2 illustrates by means of a partial schematic representation a variant embodiment of the service tube of FIG. 1, supplying each V of the impacting plate. 

1-3. (canceled)
 4. A device for cooling food products by impacting jets comprising a tunnel that comprises a conveyor adapted to convey the food products in the tunnel at least one plate, situated facing the conveyor, and provided with a plurality of orifices traversing through the at least one plate, the at least one plate being situated above and/or below the conveyor; and at least one blowing system adapted to blow a cold fluid through the plurality of traversing orifices and towards the food products, the device comprising a network of channels for distributing a blow gas above a top plate that is provided with the plurality of orifices traversing through the top plate and/or below a bottom plate that is provided with the plurality of orifices traversing through the bottom plate, one or more of the channels being provided with injection orifices or injection tubes, the positioning of which is adapted to direct the blow gas towards the plurality of orifices traversing through the top and/or the bottom plate, in order to simultaneously clear two or more of the plurality of orifices traversing through the top and/or the bottom plate.
 5. The device of claim 4, wherein the plate or plates are in the form of a corrugated stainless sheet in the form of successive Vs, each V being provided with a row of traversing orifices, and in that the network of channels comprises at least one service tube, associated with each plate, closed at one of the ends thereof, housing one of the ends of the Vs, and in that the device comprises, facing each V, an injector connected to the service tube and adapted to inject the blow gas into the V corresponding thereto.
 6. A method for cooling food products by jets blasting in a tunnel that comprises device for cooling food products by impacting jets comprising a tunnel that comprises a conveyor adapted to convey the food products in the tunnel at least one plate, situated facing the conveyor, and provided with a plurality of orifices traversing through the at least one plate, the at least one plate being situated above and/or below the conveyor; and at least one blowing system adapted to blow a cold fluid through the plurality of traversing orifices and towards the food products, the device comprising a network of channels for distributing a blow gas above a top plate that is provided with the plurality of orifices traversing through the top plate and/or below a bottom plate that is provided with the plurality of orifices traversing through the bottom plate, one or more of the channels being provided with injection orifices or injection tubes, the positioning of which is adapted to direct the blow gas towards the plurality of orifices traversing through the top and/or the bottom plate, in order to simultaneously clear two or more of the plurality of orifices traversing through the top and/or the bottom plate, wherein the method comprises the step of injecting the blow gas through the plurality of traversing orifices, at least one of the traversing orifices having a frost accumulation, to thereby reduce or eliminate the frost accumulation. 